Far-infrared light has a wavelength band ranging from 8 to 12 μm and is emitted, for example, from humans and animals in the form of heat, that is, in the form of infrared radiation. In view of the fact, far-infrared light is used in imaging in a dark place, observation of a temperature distribution, and other similar applications.
An optical system for collecting far-infrared light includes no glass lens for collecting typical visible light due to its low transmittance for far-infrared light but in many cases a lens made of germanium (Ge) or any other material that transmits a sufficient amount of infrared light. Since germanium has a high refractive index of about four, the surface reflectance is high but absorptance is nearly zero, which allows a high transmittance of 90% or higher to be achieved when an appropriate antireflection film is coated.
Germanium is, however, very expensive because it is a rare mineral.
Lens materials that are less expensive but have lower transmittance than germanium include silicon (Si), zinc sulfide (ZnS), zinc selenide (ZnSe), chalcogenide glass, which is a compound of chalcogen and germanium, and other crystalline materials.
Although these materials are inexpensive, they are disadvantageously very hard, as in the case of Ge. That is, since they are very hard, it takes long hours to process them and it is hence difficult to reduce the cost in some cases. In particular, to polish any of the materials into an aspheric shape, it is necessary to use a precise manufacturing apparatus for long hours, typically resulting in an increase in cost.
Press working of zinc sulfide (ZnS) and chalcogenide glass is under investigation, but a lens or an optical system for far-infrared light has not yet been produced at low cost so far.
JP-A-2010-39243, JP-A-2009-63942, and JP-A-2008-128913, for example, describe infrared optical systems of related art.
For example, JP-A-2010-39243 discloses an optical system using three Ge lenses. The optical system shows excellent optical characteristics within a viewing angle of 30 degrees or greater.
Further, in JP-A-2010-39243, using spherical lenses allows the processing cost to decrease.
However, since Ge is a very expensive material in the first place, an inexpensive device is not achieved.
In JP-A-2009-63942, ZnS, the material cost of which is less expensive than Ge, is used. To reduce a greater amount of aberration resulting from the fact that ZnS has a refractive index lower than that of Ge, aspheric surfaces are employed. As a result, difficulty in processing aspheric surfaces and a long processing period make it difficult to reduce the cost.
In JP-A-2008-128913, which discloses a case where a polyethylene lens is used to correct aberrations and a silicon lens is used to collect light, since the polyethylene lens is disposed in a position closest to an object, the polyethylene lens is inevitably degraded by an external force and ultraviolet radiation. Further, since the optical system is not symmetric with respect to an aperture, aberration correction capability may not be sufficient.